The AC current transformer has been well-known for many decades, and is used where alternating currents are to be measured. Current transformers are often used to provide galvanic isolation between the circuit being measured and measurement or control apparatus. However, if sufficient direct current or low frequency alternating current flows in the circuit being measured, an AC current transformer, by itself, may, due to current transformer core saturation, be unable to transform a replica of its input current to measurement or control apparatus.
This invention is one form of apparatus known as direct-current current transformers, or DCCT's. Many prior art DCCT's supplement an AC current transformer to provide DC response. One common method is exemplified by the AC/DC current probe of U.S. Pat. No. 7,309,980, wherein a Hall-effect transducer provides low frequency transformation. Such devices have proven useful, but are subject to thermal drift, and to stray magnetic fields and, may require de-gaussing. Another method is alternately to saturate and de-saturate a current transformer core to convert the DC magnetic flux therein into a transformable AC signal. This method is exemplified by the current probe of U.S. Pat. No. 6,885,183, which also exemplifies another much-used method, to wit, that of using a DC feedback winding to cancel the DC magnetic flux in the transformer core. Generally, the methods using DC feedback are either complex or have bandwidth limited to a few KHz. Numerous other methods, some very complex, have been employed to provide a direct-current current transformer, or DCCT. One prior art DCCT, a single commutated-element DCCT, that of U.S. Pat. No. 8,929,053 B2 is a simple open loop device, but suffers from sub-100 nS transient disturbances that may degrade its utility at high frequencies.